In an effort to reduce power consumption and to conserve energy, many households and commercial establishments in the United States and around the world are replacing conventional, energy-demanding light sources such as incandescent and halogen lights with more efficient LED sources (e.g., LED lamps, modules, etc.). As with conventional lights, dimmers are commonly used with the LED-based fixtures, allowing a user to choose a desired level of brightness. In some configurations, a single dimmer may be used to adjust the brightness of more than one LED source, and the user may expect the brightness of the different LED sources to be substantially similar, according to the setting of the dimmer.
A dimmer generally dims a light source (i.e., controls its brightness) by controlling the average power supplied to the light source. To achieve this, a phase dimmer receives the alternating current (A/C) mains power signal (e.g., 110 VAC, 230 VAC, etc.) and chops it, i.e., sets the signal value at or near zero for a certain duration, also called a phase. The larger the phase of the A/C mains signal chopped, the less power will be supplied by the dimmer's output signal. Therefore, the brightness of the light source, which is proportional to the average power received, is inversely proportional to the phase of the dimmer's output that is chopped.
The A/C mains power signal, whether chopped by a dimmer or unchopped, is often unsuitable for use by a LED and/or conventional light source (e.g., halogen lamp). Therefore, a transformer is used to condition the A/C mains or dimmer output signal into a suitable power signal. Typically, a transformer receives an input signal having a certain root-mean-squared (RMS) voltage (e.g., 110 VAC, 230 VAC, etc.) and generates an output signal having a similar “envelope” but a different (usually lesser) RMS voltage (e.g., 12 VAC, 24 VAC, etc.).
The envelope of a signal waveform is the shape obtained by connecting the signal peaks and ignoring the instantaneous signal values between successive peaks. A transformer output signal having a similar envelope as that of the input signal generally implies that when the input signal is at or near zero the envelope of output signal is also at or near zero. When the input signal is at or near its peak, the envelope of the output signal is also at or near its peak (although the two peak values may be different), and when the input signal is chopped, so is the output signal. As a result, the average power supplied by the transformer to the light source is adjusted according to the dimmer setting.
Some transformers do not generate an output signal having a similar envelope because they do not “trigger,” i.e., initiate power conditioning, immediately after the signal crosses the zero level. Instead, these transformers trigger after a short delay, usually on the order of a few hundred microseconds. As a result, the transformer output remains substantially zero during a certain phase in addition to the phase during which the output is chopped in response to the dimmer setting. Thus, the average power delivered by a transformer is determined not only by the dimmer setting but also by the instant at which the transformer triggers. As a result, the brightness of the light source powered by the transformer is slightly less than that required by the dimmer setting due to the additional chopping caused by the delayed triggering of the transformer.
Furthermore, different transformers may trigger at different instants, and hence, the time period other than the chopping phase during which the transformer output remains substantially zero can be different for different transformers. As a consequence, the average power supplied by each transformer in response to a certain dimmer setting can be different. This may cause the brightness of various LED sources to be substantially different for the same dimmer setting, which may be inconsistent with the user's expectation. Accordingly, there is a need for a brightness controller that adjusts the brightness of one or more LED and/or other lights with substantial uniformity for a given dimmer setting.